The present invention relates to systems for selecting a transmission control procedure or protocol suitable for communication of data via an integrated services digital network (ISDN) between a computer and a plurality of terminals.
In order to ensure transmission of data to a target terminal, it is necessary to confirm the connection of a line, the target terminal and the correct reception of data. A series of these procedures is referred to as transmission control, which can be said to be a rule in data transmission. The transmission control includes line control directed to the connection of a communication line to a target terminal, control for the synchronization of a digital signal, error control for error detection and execution of the transmission control procedure. The transmission control procedure includes setting of a logical data link for a communication program on the line between the transmission end and the reception end and transmitting data in accordance with a predetermined operational procedure. Usually, the transmission control procedure varies depending on the kind of a terminal to which the data is transmitted. The kind of transmission control procedures are, for example, standardized in accordance with CCITT Recommendations.
Conventionally, various transmission control procedures are used between a computer (center) and terminals. Generally, one terminal installs one kind of transmission control procedure while the computer (center) uses a communication control device which includes transmission control circuits to perform a plurality of kinds of transmission control procedures to communicate with various terminals.
When a terminal calls and communicates with the computer via a switching network, it is required to cause the transmission control procedure used by the terminal to coincide with that used by the computer. To this end, a plurality of circuit lines are conventionally connected to the communication control devices of the computer and fixedly allocated corresponding specific transmission control procedures. Namely, the lines and the transmission control procedures are put in one-to-one corresponding relationship. When a terminal sends a signal on a line, the operator dials a telephone number of the line to which the same transmission control procedures as that used by the terminal is allocated to thereby connect the terminal and the line having the same transmission control procedure.
The conventional communication control devices of the above computer are disclosed, for example, in Japanese Patent Publication JP-A-56-94861 filed by FUJITSU Limited, Dec. 28, 1979 in Japan; Japanese Patent Publication JP-A-62-2749 filed by NEC Corporation, Jun. 28, 1985 in Japan: and Japanese Patent Publication JP-A-60-162334 filed by HITACHI Limited, Feb. 3, 1984 in Japan.
These conventional systems are each effective for selecting a specific transmission control procedure using a telephone number in a communication system which uses an analog switching network such as that shown in FIG. 1 or for a communication control apparatus having a single channel, but cannot be applied to an ISDN which will be the main flow of the network infrastructure in the future.
According to the ISDN, a plurality of different terminal devices, for example, telephones, facsimile terminals, video signal devices, and personal computers can communicate signals collectively on a single digital line. The ISDN lines include a plurality of B channels of a 64 kb/s rate and a D channel of 16 or 64 kb/s. The B channel is a data channel to transmit communication data therethrough from a terminal while the D channel is a control channel to transmit a signal for transmission control therethrough.
As shown in FIG. 2, a plurality of communication channels are multiplexed in a single interface 6 (two channels are multiplexed in the basic interface in the B channel and 23 or 24 channels in the primary-rate group interface) in the ISDN and communication is possible with different terminals using the respective channels. Since only one telephone number is given to a single interface, it is impossible to designate, using a telephone number, the line of the communication control device to which the same transmission control means as that which the terminal affords is assigned.